In the present invention, "%" for hydrogen concentration means "% by volume" here and hereinafter.
The continuous heat treatment furnace is, basically, a facility for applying heat treatment of a predetermined heat pattern while continuously passing strip-like materials such as steel strips, which is constituted by successively disposing furnace zones each having a processing performance of heating/soaking/cooling (slow cooling and rapid cooling) in the order of treatment.
For example, a continuous heat treatment furnace for a cold-rolled steel strips comprises, as shown in FIG. 4, a heating zone 10 for heating a steel strip S to a predetermined temperature, or further soaking or further slowly cooling the same, a rapid cooling zone 11 for rapidly cooling in a predetermined temperature range and a cooling zone 12 for cooling it to a predetermined treatment completion temperature or averaging it before cooling, arranged and constituted in the order of treatment.
If the surface of materials is oxidized during heat treatment, the appearance of the products is deteriorated, so that the inside of the continuous heat treatment furnace is controlled to a non-oxidative atmosphere. In a continuous heat treatment furnace for steel strips, a mixed gas (HN gas) of a hydrogen gas and a nitrogen gas containing several % of hydrogen gas is generally used as an atmospheric gas.
When such HN gas is used, hydrogen contributed to reduction is consumed and formed into H.sub.2 O along with the progress of the heat treatment, and the atmosphere inside the furnace can no more be kept to a non-oxidative state. Therefore, a discharge pipe and a supply pipe for the atmospheric gas are disposed to each of the furnace zones to discharge spent gases and supply fresh gases thereby keeping a predetermined hydrogen concentration in the furnace.
By the way, the composition of the atmospheric gas is not always identical for every furnace zone but, as described below, a composition of atmospheric gas different from others is sometimes adopted in a certain furnace zone depending on the characteristics to be provided to steel strips.
For example, for low carbon steel having a C content of from 0.01 to 0.02 wt %, a so-called overaging treatment of heating, soaking and then rapidly cooling a steel strip to solid-solubilize C in the steel to supersaturation and then keeping it at about 400.degree. C. is conducted in order to improve the aging property. Rapid cooling technique in this case can include a gas jet cooling method of cooling/recycling an atmospheric gas by a heat exchanger, and blowing it as a high speed gas jet stream from gas jet chambers 13 as shown in FIG. 4 to a steel strip, a roll cooling method of urging a cooling roll having coolants filled therein to a steel strip and a water cooling method or a mist cooling method of blowing water or mist to a steel strip. Among them, the gas jet cooling method can provide satisfactory appearance and shape to the steel strip after cooling and is less expensive in view of facilities compared with other methods.
However, the gas jet cooling method has a drawback of low cooling rate. In order to overcome the drawback, Japanese Patent Examined Publication Sho 55-1969, Japanese Patent Unexamined Publication Hei 6-346156 and Japanese Patent Unexamined Publication Hei 9-235626 have disclosed the use of an HN gas having a cooling performance enhanced by increasing a hydrogen concentration in a rapid cooling zone. Then, satisfactory rapid cooling at a cooling rate over 50.degree. C./s is possible in the rapid cooling zone.
When using an atmospheric gas in a certain furnace zone different from that in other furnace zones, it is necessary to avoid mixing with atmospheric gases from those of other furnace zones. Therefore, sealing means are disposed at the boundary with other furnace zones.
Concrete structures or devices for known sealing means can include, for example, (A) a plurality of partition wall structures which also serve as processing chambers disposed to the boundary between each of atmospheric gases of different compositions and capable of supplying/discharging the atmospheric gases of different compositions (Japanese Patent Unexamined Publication Hei 5-125451), (B) a device for sliding contact of a seal member with a steel strip (Japanese Utility Model Examined Publication Sho 63-19316), (C) a device comprising a combination of sealing rolls, blow nozzles and sealing dampers (Japanese Patent Unexamined Publication Sho 59-133330), and (D) a roll-sealing device 4 comprising rolls rotating at the same speed as the passing speed of a material while putting the material between them from the front and back surfaces of the material as shown in FIG. 4. Further, in a rapid cooling zone 11 of FIG. 4, a roll-sealing device 4 is disposed not only to the entrance and the exit but also to the exit at the upstream of the rapid cooling zone in which gas jet chambers 13 are disposed.
Among such sealing means, scratches are caused to the steel strip by contact with the sealing member in (B). This risk is particularly large under heat treatment condition of high passing speed. In (A) and (C), a consumption of atmospheric gas is worsened, since the flow rate of the sealing gas has always to be kept and, in addition, a gas flow rate at high accuracy is necessary for ensuring the sealing performance, to make the facility expensive. On the contrary, no scratches are caused to steel strips and the facility is inexpensive in (D).
As described above, in the rapid cooling zone of the continuous heat treatment furnace, it is advantageous to adopt a gas jet cooling method of using an HN gas at a higher hydrogen concentration than that in other furnace zones (heating zone, cooling zone or the like) and recycling/cooling and blowing the gas to the steel strips in view of the surface property of products and the cost for facilities. It is advantageous to adopt the roll-sealing device as the sealing means with the same viewpoint.
However, as actually shown in FIG. 4, when roll-sealing devices 4 are disposed before and after (at the entrance and exit) of the rapid cooling zone 11 to completely shield the atmospheric gas at high hydrogen concentration in the rapid cooling zone, a dynamic pressure is generated by the stream formed by the atmospheric gas at high hydrogen concentration blown to the strip material and flowing along the strip-like material in the rapid cooling zone (also called as an entrained stream). The dynamic pressure thus generated is interrupted by the roll-sealing devices to result in elevation of a static pressure in the vicinity of the roll-sealing devices. For example, FIG. 5 shows the result of measurement for the static pressure (FIG. 5(a)) and the hydrogen concentration in the atmospheric gas (FIG. 5(b)) at points P1 to P9 in the rapid cooling zone and before and after the zone when a strip material having a 0.8 mm thickness and a 1250 mm width is passed through the continuous heat treatment furnace at a line speed of 400 mpm. As can be seen from FIG. 5(a), large static pressure gaps are caused at some points. Therefore, the balance of the furnace pressures is lost in the rapid cooling zone and before and after of the zone to cause large gas streams, as a result, the atmospheric gas at a high hydrogen concentration in the rapid cooling zone is flown out of the rapid cooling zone, and the hydrogen concentration in the rapid cooling zone is lowered as shown in FIG. 5(b). It is necessary to increase the amount of the HN gas at a high hydrogen concentration to be charged in order to compensate the lowering of the hydrogen concentration in the rapid cooling zone, which results in worsening of the RN gas consumption.
After all, provision of a strong sealing device in order to prevent the gas flow leads to an unintentional result of inducing the gas flow due to the distribution of the furnace pressure (atmospheric pressure inside the furnace). Such problems are not taken into consideration in existent sealing means.
In addition, it has been found by the recent study of the inventors that the discharge of the atmospheric gas at high concentration from the rapid cooling zone not only leads to the worsening of HN gas consumption but also gives an influence on the crystal structures of the strip-like material during recrystallization upstream to the rapid cooling zone. Namely, it has been obtained such a finding that if the hydrogen concentration in the furnace zone in adjacent with the inlet of the rapid cooling zone is increased to higher than 10%, nitridation proceeds at the surface layer of the strip material in a state of a high temperature before rapid cooling, resulting in a problem of causing partial hardening to the surface layer.
In view of the foregoing problems of prior art, an object of the present invention is to provide a continuous heat treatment furnace having a rapid cooling zone of a high hydrogen concentration, capable of properly controlling the hydrogen concentration of an atmospheric gas in a furnace zone for heating and keeping after heating and the hydrogen concentration in the atmospheric gas in the rapid cooling zone, and excellent in the HN gas consumption, by preventing mixing between the atmospheric gas at high hydrogen concentration in the rapid cooling zone and the atmospheric gas in the zones in adjacent with the rapid cooling zone a (heating zone, cooling zone and the like) of a gas jet cooling system.
Disclosure of the Invention
The present invention provides a method of controlling an atmosphere in a continuous heat treatment furnace of heat-treating a strip-like material in an atmospheric gas, heating the strip-like material in the course of the treatment and then rapidly cooling it by blowing a hydrogen-containing gas, wherein the hydrogen concentration in the atmospheric gas in the furnace zone for heating the strip-like material and the furnace zone for keeping it after the heating is controlled to 10% or lower (first invention).
The present invention also provides a cooling method of heat-treating a strip-like material in an atmospheric gas, heating the strip-like material in the course of the treatment and then rapidly cooling it by blowing a hydrogen-containing gas, wherein the hydrogen gas concentration of the atmospheric gas in the furnace zone for heating the strip-like material and a furnace zone for keeping it after heating is controlled to 10% or lower, the tension per unit cross section of the material: Tu (kgf/mm.sup.2) is kept within a range capable of satisfying the following conditions (formula corresponding to any one of the formulae (1) to (3)) depending on the thickness t (mm), the width W (mm) of the strip material, and a hydrogen-containing gas at a hydrogen concentration of 10% or higher is blown to the material (second invention).
Note
(a) Under the condition: W&lt;1350 mm EQU 1.88-0.18.times.t-0.00080.times.W.ltoreq.Tu.ltoreq.2.38-0.11.times.t-0. 00084.times.W (1) PA1 (b) Under the condition: W.gtoreq.1350 mm and t.ltoreq.0.85 mm EQU 0.73+0.38.times.t-0.00030.times.W.ltoreq.Tu.ltoreq.1.23+0.35.times.t-0. 00028.times.W (2) PA1 (c) Under the condition: W.gtoreq.1350 mm and t&gt;0.85 mm EQU 1.10-0.00033.times.W.ltoreq.Tu.ltoreq.1.54-0.00029.times.W (3)
Further, the present invention provides a continuous heat treatment furnace having a plurality of furnace zones arranged successively for the heat treatment of a strip-like material in an atmospheric gas, wherein one of the furnace zones except for the first and last zones is a rapid cooling zone for rapidly cooling the material by blowing an atmospheric gas, which comprises a first roll sealing device at an entrance and a second roll sealing device at an exit as atmospheric gas sealing means, and in which the inlet of the first roll sealing device and the outlet of the second roll sealing device are connected (third invention).
The present invention also provides a continuous heat treatment furnace having a plurality of furnace zones arranged successively for the heat treatment of a strip-like material in an atmospheric gas, wherein one of the furnace zones except for the first and last zones, is a rapid cooling zone for rapidly cooling the material by blowing an atmospheric gas, and comprises a roll-sealed chamber partitioned by first and second roll sealing devices from the upstream at an entrance and a third roll sealing device at the exit as atmospheric gas sealing means, in which the roll-sealed chamber and an upstream portion in the rapid cooling zone are connected (fourth invention)
The prevent invention also provides a continuous heat treatment furnace having a plurality of furnace zones arranged successively for the heat treatment of a strip-like material in an atmospheric gas, wherein one of the furnace zones except for the first and last zones is a rapid cooling zone for rapidly cooling the material by blowing an atmospheric gas, and comprises a roll-sealed chamber partitioned by first and second roll sealing devices from the upstream at the entrance and a third roll sealing device at the exit as atmospheric gas sealing means, in which the inlet of the first roll-sealing device and the outlet of the third roll-sealing device are connected, and the roll-sealed chamber and an upstream portion in the rapid cooling zone are connected (fifth invention).
The present invention further provides an invention as defined in any one of third to fifth inventions wherein bridle rolls are disposed before and the after the rapid cooling zone (sixth invention).